Wireless communication systems are widely deployed to provide various types of communication, such as voice and data communications. These systems may be based on a variety of modulation techniques, such as frequency division multiple access (FDMA), time division multiple access (TDMA), and various spread spectrum techniques. One common spread spectrum technique used in wireless communications is code division multiple access (CDMA) signal modulation. In CDMA, multiple communications are simultaneously transmitted over a spread spectrum radio frequency (RF) signal. Some example wireless communication devices (WCDs) that have incorporated CDMA technology include cellular radiotelephones, PCMCIA cards incorporated within portable computers, personal digital assistants (PDAs) equipped with wireless communication capabilities, and the like. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards.
Other wireless communication systems may use different modulation techniques. For example, GSM systems use a combination of TDMA and FDMA modulation techniques. These techniques are also used in other systems related to GSM systems, including the DCS1800 and PCS1900 systems, which operate at 1.8 GHz and 1.9 GHz, respectively.
Some communication systems are not yet widely deployed. For example, costs associated with deploying W-CDMA systems have generally limited the coverage of such systems to large cities. For such systems, it is desirable to use a secondary communication system to provide coverage in areas in which there is no W-CDMA coverage. For instance, W-CDMA systems often use a network of GSM carriers to fill in gaps in coverage areas. Other systems may use different secondary communication systems, such as IS-95 networks.
When a WCD is in a W-CDMA call and the W-CDMA signal becomes weak, the WCD may perform a handover to a GSM carrier. For example, a handover from WCDMA to GSM typically occurs when the WCD leaves an area with W-CDMA coverage. The handover decision may be based on a number of handover measurements, including, for example, received signal code power (RSCP), signal-to-interference ratio (SIR), or a received signal strength indicator (RSSI). The WCD may perform handover measurements in a compressed mode in which the downlink transmission contains breaks. During these breaks, the WCD may monitor one or more GSM carriers by obtaining GSM signal strength measurements. Under appropriate circumstances, the WCD switches the call to a selected GSM carrier until the WCD enters an area with WCDMA coverage, at which point the WCD switches the call back to the W-CDMA system.
In CDMA, GSM, and other wireless communication technologies, frequency tracking loops are often used to monitor the frequency of received signals and adjust the signals accordingly. In particular, frequency errors or variations often exist in the carrier signals received over forward or reverse links of the system. A forward link, sometimes referred to as a downlink, refers to a signal sent from the base station to a wireless communication device. A reverse link, sometimes referred to as an uplink, refers to a signal sent from the WCD to the base station.
There are generally two main causes of the errors that can contribute to unwanted frequency variation of a carrier signal. One relates to what is commonly known as the Doppler effect or Doppler shift. The Doppler effect manifests as a change in the frequency of a received signal due to a relative velocity between the transmitter and the receiver. For example, if a WCD is moving away from the base station as it transmits a signal over the reverse link, the base station receiver will receive a signal that has a lower frequency, i.e., a longer wavelength, than the transmitted signal. Conversely, if the WCD is moving toward the base station as it transmits, the base station receiver will receive a signal that has a higher frequency, i.e., a shorter wavelength, than the transmitted signal. Because WCDs are often used within vehicles or high speed transit systems, correcting for Doppler shifts can be an extremely important factor in maintaining a robust and effective wireless communication system.
Another cause of error that can contribute to unwanted frequency variation relates to variations between local clocks of the various devices in the wireless communication system. Each device in the system typically produces carrier signals using a frequency synthesizer that uses the local clock of the device as its timing device. Each local clock, however, typically has an unknown timing error. WCDs often employ relatively low-cost local clocks, such as voltage-controlled, temperature-compensated crystal oscillators (VCTCXOs). These local clocks can introduce significant frequency errors in the carrier signal.
Frequency tracking loops correct for frequency errors by estimating the frequency errors and adjusting the frequency of received signals. For example, a frequency discriminator can be used to compute an estimate of the frequency error. The frequency discriminator calculates residual frequency errors and continuously accumulates the residual frequency errors to estimate the actual frequency error. A rotator then uses the accumulated estimate to adjust the frequency of the received signal accordingly, thus reducing the residual frequency error. The residual frequency error eventually converges to approximately zero, such that the accumulated estimate is approximately equal to the actual frequency error. In this manner, a feedback loop can correct for frequency errors in a received carrier signal.